Field of the Invention
The present invention concerns a process for oxidizing lignin at alkaline conditions. This oxidized lignin can be used as a dispersant in various end-products, for example as a renewable plasticizer in cement-containing products, such as concrete, where the ability of the oxidized lignin to endure alkaline conditions is an advantage.
Description of Related Art
Concrete is one of the most commonly used industrially manufactured materials in the world, and manners of improving its strength and durability are constantly under research. Aggregate and cement are the main components of concrete, but also water is required, and usually, various additives are used, for example to improve the fresh-stage or hardening properties of the final product.
Water plays a dual role in concrete production; it provides the concrete mixture with certain rheological properties, and it participates in the essential hydration reaction, and is required to bring the cement into a paste-like form. However, concrete is particularly strong when it contains only low amounts of water. Thus, a lower water-to-concrete ratio yields a stronger, more durable concrete, while a higher ratio gives a freely-flowing cement-mixture with a higher slump.
To obtain both a strong concrete product and advantageous cement flow-properties, plasticizers (or dispersants) are often added into the concrete mixture to improve the workability, i.e. to decrease the yield stress (or shearing resistance), of the concrete mixture, while using low water contents.
Petroleum-based polyelectrolytes, known as superplasticizers, have typically been considered to be the most efficient plasticizers. However, there exists a constant need to replace petroleum-based products with renewable alternatives.
Superplasticizers act by adsorbing to the cement particle surface and cause electrostatic and steric repulsions between individual cement particles to achieve full dispersion. The active functional groups of the superplasticizers, i.e. the sulfonic (—SO3H) and the carboxylic (—COOH) groups, are responsible for achieving the electrostatic repulsion by interacting with the active sites on the cement particles, while the branched structure of the polyelectrolyte achieves steric repulsion by forcing the cement particles apart.
Water-soluble lignosulfonates, which are by-products of sulfite pulping, are also used due to their highly branched and sulfonic charged structure, but their effect is limited.
At the present, the main source of lignin is the alkaline sulfate (i.e. kraft) process. The soda process, on the other hand, is the predominant process used for chemical pulping of non-wood materials. Also 2nd generation bioethanol production from lignicellulosics provides a source of lignin.
Presently only a fraction of the lignin formed during the pulping is separated from the spent pulping liquors and employed in specialty products. Instead, the remaining liquors (and lignin) serve, for example, as biofuel in pulp mills. Thus, there is a clear need for further lignin exploitation methods.
Kraft or soda lignins (among other potential lignins) would be both cost-effective and environmental alternatives to petroleum-based or lignosulfonate-based plasticizers in concrete mixtures. In unmodified form they, however, lack the sufficient efficiency required for their use as plasticizers or dispersants.
Modification of lignin has been attempted in the past, such as by sulfonating the lignin (as in CN102241818A, CN101885823A and U.S. Pat. No. 5,811,527A), or by adding amine or ammonium groups to lignosulfonates (as in U.S. Pat. No. 6,238,475B1 and WO2001036344A2). Such modifying processes generally include also an oxidizing step, but this step is not carried out under effective conditions, since it is merely intended to provide a suitable basis for further modifications.
This known oxidation of lignin has been described in, for example, U.S. Pat. No. 3,910,873A, but this process was not developed for the requirements of the concrete industry, and can still be improved. Further processes for the oxidation of lignin are described in U.S. Pat. No. 4,611,659A and U.S. Pat. No. 3,726,850A,